What is a NAMUR proximity sensor?

What is a NAMUR proximity sensor?  In this article, we introduce the sensors that called NAMUR proximity sensors. We discuss how this NAMUR sensor differs from other proximity sensors, also what are their applications in the various industries.

The term “NAMUR” is an acronym for a very long German description:
“Normen Arbeitsgemeinschaft für Mess- Und Regeltechnik in der chemischen Industrie”
The schematic symbol for a NAMUR proximity sensor has illustrated in the figure1.

A NAMUR

A NAMUR sensor is a typical proximity sensor because they look very much alike. But physical shape and appearance are the only characteristics they have in common.There are different reasons why you may need a NAMUR sensor, we will explain some of them here.NAMUR proximity sensors can be classified into capacitive, inductive, magnetic or photoelectric sensors.

Electrical characteristics of a typical proximity sensor is the same as a switch.

In other word, a proximity sensor acts just like a simple switch. When the target is away from the sensor, the switch is open. When the target is close to the sensor, the switch is closed.

A NAMUR sensor is a special type of proximity sensor. NAMUR sensors use a two-wire connecting cable instead of the typical three wires used by standard PNP or NPN sensors.

Standard three wire DC sensors use two wires for power and the third for the output, they have either a PNP or a NPN transistor output that switches the sensor if it detects an adequate target. You may be asking yourself why, since NAMUR sensors have so many interesting features, are not used everywhere. The answer is that although NAMUR sensors have a useful set of features, they require a switching amplifier to work with a typical DI I/O PLC card, or I/O card specifically designed to work with NAMUR sensors that integrates the switching amplifier in the card’s electronics.

That additional equipment quickly increases the costs. Three wire sensors have become commodity level products, so they have lower prices and do not require additional hardware to work properly.

But NAMUR sensors have their market niche: application in classified areas as potentially explosive due to the presence of flammable gases and vapors.
The NAMUR proximity sensor has been in use for many years but like any electronic device, there are many different symbols in use.
Briefly, NAMUR proximity sensors were developed to ensure safety in hazardous locations.
NAMUR proximity sensors are used in classified hazardous location areas that have explosive gas, dust, or fibers present.

If you need to install a proximity sensor in a classified area that contains a potentially explosive atmosphere, you cannot use a standard three wire sensor, because when the sensor switches to indicate a detection, the signal wire is connected or disconnected to the power supply, that action can generate an arc thus transforming the sensor into a potential source of ignition.

A NAMUR sensor is a non-amplified sensor that supplies two different signal levels depending upon switch state. These signal levels are low-level current. A logic interface circuit, controller, PLC, DCS, or intrinsically

safe (IS) barrier is needed to read these current levels. A NAMUR output sensor is required in installations where special safety measures are necessary (locations with explosion hazard, or personal safety).

NAMUR sensors are designed to be powered by a switching amplifier that limits the available voltage to 8.2 V. This voltage is kept constant by using two different current levels to indicate if the sensor is detecting an adequate target or not. These current levels are usually 1 mA with a target present and 3 mA without one.
NAMUR proximity sensors limit current so that the current is not capable of causing ignition in hazardous atmospheres.
A NAMUR output sensor can be normally open or normally closed, but usually it’s normally closed. This output is solid state, so there will be some current level on the output whether it’s on or off.

When you look at a NAMUR output sensor data sheet, it will list nominal voltage (8.2 V), load (1 k ohm), and current for measuring plate detected (<1 mA) and not detected (>2.2 mA) (for normally closed output). Because of the low-current switching level, you’ll need an amplifier or an input that is designed specifically for NAMUR. Although the sensor may be able to handle a range of voltages (7 VDC to 12 VDC or 5 VDC to 25 VDC), the voltage level should be at 8.2 VDC with 1k ohm resistance as the load.
Typical proximity sensors that continually make and break an output circuit could create sparks that could cause an explosion.
A NAMUR proximity sensor has a switching circuit similar to a typical proximity sensor, but instead of making and breaking an external circuit, the switching circuit is used to change the output current flow by altering an internal resistor network.
Using this technology, there are 2 output current values depending upon the position of the target.
There are 4 NAMUR proximity sensors used to measure the shaft speed on four separate chemical pumps.
The pumps are located in a hazardous area, therefore typical proximity/speed sensors are not suitable. NAMUR sensors are manufactured with either metallic, conductive housings or with plastic, non-conductive housings. Metallic housings have better mechanical characteristics and have usually longer life than plastic ones, but they offer less resistance to water ingress or corrosion. So, for environments with elevated levels of humidity or that demand high IP ratings, the use of NAMUR sensors with plastic housings is preferred.

But since plastics are basically nonconductive materials, certain environmental conditions (such as dry, dusty atmospheres or any condition that may generate electrostatic charges in the metallic parts, and most importantly, in the plastic parts) can make an intrinsically safe NAMUR sensor work as an electrostatic ignition source.

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